1. Field of the Invention
The invention is in the field of display systems and concerns a display system with a light source.
2. Description of Related Art
Display systems that include light sources have widespread applications. They include rear projection displays (including screens such as monitor screens and television screens), front projection systems such as LCD, DLP, LCOS (etc.) front projectors, and pocket projectors, head-up displays, etc.
A display system projects an image on a surface (for example a panel) or directly into the human eye. An illumination system generates broadband light suitable to be detected by an appropriate photo-detector to illuminate the environment (for example the human eye). Light sources normally used in projection and display systems are broadband to allow a large color gamut and incoherent to avoid interference between different optical paths that can result in speckle and shadows on the panel. High power level is required at the source to obtain a suitable intensity on the panel, mainly due to the considerable optical loss associated with the projection system. Modern projection display systems include light modulating devices to generate the image from electronic information, like Spatial Light Modulators (SLMs), Micro-Mechanical Systems (MEMS), Liquid Crystal Displays (LCDs), Digital Micro mirror Devices (DMDs), Digital Light Projectors (DLPs), Grating Light Valves (GLVs), and Electrically Switchable Bragg Gratings (ESBGs) among others. All splitting and modulating systems have the purpose to manipulate the intensity of the illuminating beam and generate the image on the panel. This is sometimes referred to as Optically Written Display, as described in U.S. Pat. No. 6,897,999. The overall system efficiency is strongly affected by the coupling between the source and the modulating unit(s) (see: “Projection display throughput: Efficiency of optical transmission and light-source collection”, IBM Journal of Research and Development, May-July 1998). Therefore, good beam quality at the light source is required for an efficient energy conversion of the overall system or intensity manipulation.
A typical projection display is made out of the following functional parts:                A broadband light source, such as high power arc lamp;        A filter to remove ultra-violet and infra-red components of the spectra (if required by the source and the rest of the optical system);        Polarizers to polarize the light along one axis (if required by the modulating unit);        A beam splitter, to divide light into the main colors;        One or more light modulating unit(s) defining the intensity of the image in each part;        A beam mixer to recombine the colors and form the white image;        Projection lenses to focus the light on a screen.        
Due to the complexity of the optical system, the overall power efficiency is usually very low (see: F. E. Doany et al., IBM Journal of Research and Development, vol. 42, no. 3/4, May/June 1998). The conversion efficiency of a system containing a modulating device is strongly affected by the source-to-modulating unit coupling. Modern projectors use xenon lamps as broadband light sources. The generated light is directed towards the optical system using a parabolic reflector. Despite this, the maximum coupling efficiency between the source and the first lenses is usually around 80%. Moreover, arc lamps are ultra-broadband light sources, meaning that the usable part of the spectra is narrower than the natural emission spectra of the source and a considerable amount of energy is lost filtering both the UV- and IR-components to prevent the damage of some optical parts inside the system. The following table shows the optical transmission efficiency of different steps in state of the art projection systems:
EfficiencyUV-IR filtering0.9Illumination0.85Polarization0.38Prism and Filter0.7Projection Lens0.9SLM0.6Total0.1
Some modulating devices are sensitive only to one polarization, as, for example, first generation LCD. Therefore, in these systems a considerable amount of the energy is also lost in the polarizer. Today, many displays use the Digital Light Processor (DLP), consisting of a composite mirror where each element can switch on and off very fast to change the light intensity in each part of the image. This modulating device is insensitive to the light polarization and gives better efficiency compared to polarization dependent modulating units (see: Lars A. Yoder, “An Introduction to the Digital Light Processing (DLP™) Technology”, Texas Instruments white paper).
For such display systems, various light sources are on the market, including:                incandescent bulbs;        gas discharge lamps;        xenon lamps;        cathode-ray tubes;        ultra-high pressure (UHP) lamps.        
Today incandescent bulbs and gas discharge lamps are typically used for illumination systems only. Incandescent bulbs also do not feature extremely long lifetime figures, but the low cost of each item keeps the total cost very affordable. However, the low conversion efficiency pushed some countries to ban light bulbs in aim to help cut greenhouse gas emissions. Gas discharge lamps are more beneficial in that respect but suffer from the drawbacks of limited luminance and limited spectral width.
For projection systems, however, due to the mentioned low overall power efficiency, only high-power lamps are suitable. Usually, high-power xenon lamps are employed in projection display systems to ensure suitable light intensity on the display panel. While bulbs are cheap, the cost of a xenon lamp is very high so that it contributes to a considerable fraction to the cost of the whole system. Lifetime of the light source is another issue. Typically, the xenon lamps have lifetime figures of approximately 3000 hours and even if they do not fail immediately after that time, the quality of the imaged formed on the display is reduced with time.
More in general, all the light sources commonly found in image projection or illumination systems have some well-established characteristics:                high optical power, to allow the illumination of a large area;        low energy conversion figures, meaning considerable residual heat generated to produce the required amount of light;        incoherent time evolution of the electromagnetic radiation, to avoid any interference between different optical path that can result in fringes or speckle in the image formed on the panel or the illuminated surface;        broadband emission spectra to cover the whole spectral response of the photo detector used (for example of the human eye);        beam characteristics not suitable to ensure good coupling with the other optical elements, forcing the manufacturers to use lenses, mirrors, parabolic deflectors and other optical elements to ensure directionality of the beam and good coupling with other optical elements;        reliability and lifetime issues.        